This invention relates generally to electrophotographic printing and more particularly relates to an image treatment method and apparatus for fusing color toner images to a substrate such that they exhibit uniform gloss and satisfactory color saturation properties.
In imaging systems commonly used today, a charge retentive surface is typically charged to a uniform potential and thereafter exposed to a light source to thereby selectively discharge the charge retentive surface to form a latent electrostatic image thereon. The image may comprise either the discharged portions or the charged portions of the charge retentive surface. The light source may comprise any well known device such as a light lens scanning system or a laser beam. Subsequently, the electrostatic latent image on the charge retentive surface is rendered visible by developing the image with developer powder referred to in the art as toner. The most common development systems employ developer which comprises both charged carrier particles and charged toner particles which triboelectrically adhere to the carrier particles. During development, the toner particles are attracted from the carrier particles by the charged pattern of the image areas of the charge retentive surface to form a powder image thereon. This toner image may be subsequently transferred to a support surface such as plain paper to which it may be permanently affixed by heating or by the application of pressure or a combination of both.
In order to fix or fuse the toner material onto a support member permanently by heat, it is necessary to elevate the temperature of the toner material to a point at which constituents of the toner material coalesce and become tacky. This action causes the toner to flow to some extent onto the fibers or pores of the support members or otherwise upon the surfaces thereof. Thereafter, as the toner material cools, solidification of the toner material occurs causing the toner material to be bonded firmly to the support member.
One approach to thermal fusing of toner material images onto the supporting substrate has been to pass the substrate with the unfused toner images thereon between a pair of opposed roller members at least one of which is internally heated. During operation of a fusing system of this type, the support member to which the toner images are electrostatically adhered is moved through the nip formed between the rolls with the toner image contacting the heated fuser roll to thereby effect heating of the toner images within the nip. Typical of such fusing devices are two roll systems wherein the fusing roll is coated with an abhesive material, such as a silicone rubber or other low surface energy elastomer, for example, tetrafluoroethylene resin sold by E. I. DuPont De Nemours under the trademark Teflon. To further enhance release, a release agent material such as silicone oil is applied to elastomer coating.
Conventional roll fusers are operated such that roll surface temperatures are in the order of 390.degree.-425.degree. F. Operation at these temperatures, results in shortened roll life and less reliability due to random failures such as rubber peeling off the rollers.
Following is a discussion of prior art, incorporated herein by reference, which may bear on the patentability of the present invention. In addition to possibly having some relevance to the patentability, these references, together with the detailed description to follow, may provide a better understanding and appreciation of the present invention.
"Two stage" fusing is known. For example, U.S. Pat. No. 3,861,863 discloses a black and white image fuser comprising a first stage backside heater and a second stage soft roll fuser.
U.S. Pat. No. 3,679,302 discloses first and second stage radiant fusers.
U.S. Pat. No. 3,566,076 discloses the combination of radiant and pressure fusing.
U.S. Pat. No. 4,875,611 granted Oct. 24, 1989 discloses a copy media feed system including two pair of feed rolls which are horizontally aligned to form nip areas to engage the media. Each feed roll pair comprises one drive roll and one idler roll. For one pair, the drive roll is an elastomer-covered, high friction roll and the idler roll is a hard, roll. For the second pair the drive roll is a hard, high friction roll and the idler roll is an elastomer-covered roll. This arrangement provides accurate control of the media velocity.
U.S. Pat. No. 4,791,447 granted Dec. 13, 1988 discloses a heat and pressure fusing apparatus for fixing color toner images to various types of copy substrates. The apparatus includes three roll members which cooperate to form a pair of nips. All substrates pass through a first nip and a deflector plate directs certain types of substrates through the second nip. Passage of the substrates through the first nip causes the images carried thereon to contact a conformable elastomeric surface while passage through the second nip causes them to contact a relatively rigid surface. Thus, glossy and matte color copies on substrates such as plain paper and high chroma transparencies are suitably produced in a color reproduction apparatus incorporating this fuser. Matte color copies are produced by passing the substrate through only the first nip while glossy color copies and high chroma transparencies are produced by passing the substrates through both nips.
U.S. Pat. No. 4,639,405 granted Jan. 27, 1987 discloses a method and apparatus for fixing toner images in which a copy sheet bearing unfixed toner is first passed through a pair of heated fuser rollers and is subsequently passed through surfacing rollers to provide a gloss to the toner image. In order to prevent curling of the copy sheet and blistering of the glossed image, the copy sheet is passed through a conditioner means, located between the fuser rollers and the surfacing rollers, for removing a substantial portion of the moisture from the copy sheet.
U.S. Pat. No. 4,627,813 granted Dec. 9, 1986 discloses a thermal fixing apparatus for use with a copying machine or electronic printer in which an operating temperature of the apparatus after energization is quickly reached. A pair of fixing rolls is provided, at least one of which in heated. The outer surface of the other is covered with an elastically deformable outer layer. The two rolls are pressed into abutment with one another to form a nip therebetween of the predetermined width. A plate-shaped heater element is disposed prior to the nip adjacent the path of conveyance of toner-image-bearing paper sheets to be fixed. The plane of the heater element is preferably parallel to the plane of the paper. The surface temperature of the heater element has a temperature higher at central portions than at widthwise ends thereof to provide uniform fixing conditions.
U.S. Pat. No. 4,223,203 granted Sep. 16, 1980 discloses a heat and pressure fusing apparatus for fixing toner images to copy substrates comprising a first fusing system consisting of a pair of nip forming rolls, one of which is provided with a conformable outer surface and a second fusing system consisting of a pair of nip forming rolls, one of which has a rigid outer surface. Copy substrates are passed sequentially through the nips of the first and second fusing systems, in that order such that the toner images sequentially contact the conformable outer surface and then the rigid outer surface.